Edward Smith, of Napier, asks :-

How do scientists know it is time to add a second to clocks?

Tim Armstrong, a physicist who maintains New Zealand's time standard at Callaghan Innovation, responded.

I first need to explain why we need leap seconds. The rotation of the earth is gradually slowing due to tides created by the gravitational pull of the moon. This gradual slowing is around 1 second per 18 months, but varies according to the seasons and other changes in the earth such as movements within the core of the earth.

Prior to the introduction of leap seconds global time was determined by the rotation of the earth – specifically, the mean solar time and referenced to the meridian passing through the Royal Observatory in Greenwich, London.

However, atomic clocks are far more accurate than the rotation of the earth. Since 1972, the atomic timescale called Universal Co-ordinated Time (UTC) has been the international standard for determining the second.

UTC is based on around 400 caesium clocks around the world (including two at the Measurement Standards Laboratory at Callaghan Innovation’s Gracefield site) and has an accuracy of around 1 second per 3 million years.

UTC is used by a wide range of industries which rely on accurate time measurement, from ISPs to space exploration to media outlets and in GPS for navigation.

Leap seconds were introduced (again in 1972) by the International Bureau of Weights and Measures and the International Telecommunication Union (ITU) as a means of ensuring that the time of day determined by the atomic timescale did not get too far out of sync with the length of the day.

At the time it was agreed that the two timescales would always remain in line within plus or minus 0.9 of a second by introducing leap seconds.

So, to come back to your original question, who determines when UTC is departing by around one second from the time of day? This is determined by another international organisation called the International Earth Rotation Service (IERS). It is their responsibility to collect data on the length of day from a wide variety of astronomical observations using a number of different techniques from all around the world.

Having determined that the departure between the two timescales is approaching a one second offset, they announce the introduction of a leap second. Leap seconds are only introduced at midnight UTC on 30 June or on the 31 December. The graph shows the difference between UTC and UT1 (the time of day determined by the IERS) and the introduction of the last four leap seconds during the last nine years.

There is currently a big debate going on about the continuing need for leap seconds and whether they will be stopped. One reason for stopping them is interference with computer systems, which occurred in 2012, but not during the recent leap second this year. The fate of the leap second will be on the agenda at a meeting of the ITU in November. It is possible that the recent leap second will be the last.